It is often desired to heat continuous-cast products (e.g., slabs, billets, or other workpieces) as they are conveyed along a path from one location to another. Typically, such products are conveyed by conveyor rolls, which support the product from below and are driven to impart linear motion to the product.
A typical roller induction heating line 10 for continuous-cast products according to the prior art is illustrated schematically in FIG. 1. A continuous-cast product such as a tubular workpiece 12 is conveyed from right to left as viewed in FIG. 1 by steel conveyor rolls 14 and 16. Conveyor rolls 14 and 16 are journaled for rotation in a supporting frame, and are rotationally driven, in known manner, in a counterclockwise direction as viewed in FIG. 1. The rotation of conveyor rolls 14 and 16 imparts linear movement of the tubular workpiece 12 from right to left as indicated by the large arrow at the top of FIG. 1.
As the tubular workpiece 12 is conveyed by conveyor rolls 14 and 16, it passes through an induction heating coil 18. The induction heating coil 18 is a conventional helically-wound coil known in the art. The induction heating coil 18 is excited by a high frequency ac power supply 20, also known in the art, and generates an electromagnetic field through which the tubular workpiece 12 passes. Typically, the tubular workpiece 12 is positioned so that its axis is collinear with the axis of coil 18. The electromagnetic field produced by induction coil 18 induces the flow of eddy currents in the tubular workpiece 12. The electrical resistance of the tubular workpiece 12 to the induced eddy currents results in I.sup.2 R heating of the tubular workpiece 12.
Problems arise, however, because the induction coil 18 generates a small, but non-negligible, component of the electromagnetic field perpendicular to the axis of the coil and, thus, along the axis of the tubular workpiece 12. This component of the electromagnetic field produces an electric current which flows along the axis of the tubular workpiece 12, represented by the small horizontal arrows pointing to the right in FIG. 1. This current, referred to as a parasitic current, begins to circulate along a path from the tubular workpiece 12 and into conveyor rolls 14 and 16 through a common ground, such as the supporting frame in which the rolls are journaled. This path is represented by the curved path shown below the conveyor rolls in FIG. 1. (Although the figure illustrates parasitic current flow in one direction, it will be understood that the parasitic current is an alternating current since the coil is excited by an ac power supply. ) This phenomenon causes arcing between the moving tubular workpiece 12 and the conveyor rolls 14 and 16, which causes pitting and other damage to the conveyor rolls.
Prior to the present invention, the most common way of preventing the flow of parasitic currents was to insulate the conveyor rolls from ground, in order to break up the current path. This involved cumbersome and expensive steps. One approach was to make the conveyor rolls out of ceramic. Ceramic conveyor rolls are very expensive, and can easily crack. Other techniques involved constructing the conveyor rolls from concentric steel inner and outer tubes insulated from each other by an intermediate insulator, such as a ceramic. Such conveyor rolls are extremely expensive to fabricate, and are subject to failure because of differential expansion and contraction between the steel and the insulating material when the rolls are subjected to the high temperatures involved in the continuous heating operation.
In some cases, no attempt was made to eliminate the parasitic currents. The currents were allowed to flow, and the conveyor rolls were periodically removed from the line and resurfaced to remove the pitting. Clearly, none of these approaches is very satisfactory.
The present invention provides a way of preventing the flow of parasitic currents. Consequently, the present invention prevents the damage to the conveyor rolls which parasitic currents cause, and eliminates the need for special conveyor rolls and insulating schemes to block the flow of parasitic currents. The present invention makes roller induction heating easier and cheaper than prior approaches.